Ceramic discharge vessels are generally used for high-intensity discharge (HID) lamps such as high-pressure sodium (HPS), high-pressure mercury, and metal halide lamps. The translucent ceramic vessel must be capable of withstanding the high-temperature and high-pressure conditions present in an operating HID lamp as well as be resistant to the corrosive chemical fills. The preferred ceramic for HID lamp applications is polycrystalline alumina (PCA), although other ceramics such as sapphire, yttrium aluminum garnet, aluminum nitride and aluminum oxynitride may also be used.
In conventional ceramic discharge vessels, making the hermetic seal between the ceramic vessel and the metal electrical feedthrough can be troublesome because of the very different properties of the materials, particularly with regard to the thermal expansion coefficients. In the case of polycrystalline alumina, the seal typically is made between the alumina ceramic and a niobium feedthrough since the thermal expansion of these materials is very similar. The niobium feedthrough is joined with at least a tungsten electrode which is used to form the point of attachment for the arc because of its significantly higher melting point.
Niobium however as a feedthrough material has two significant disadvantages. The first disadvantage is that niobium cannot be exposed to air since it will oxidize and the seal will fail. This necessitates that the discharge vessel be operated in either a vacuum or inert gas environment, which increases cost and the overall size of the lamp. The second disadvantage is that niobium reacts with most of the chemical fills for metal halide lamps. This concern has lead to the development of more complex electrode assemblies for metal halide applications. For example, one prior art electrode assembly for a ceramic metal halide lamp is comprised of four sections welded together: a niobium feedthrough for sealing to the ceramic arc tube; a molybdenum rod; a Mo-alumina cermet, and a tungsten electrode. Another described in U.S. Pat. No. 6,774,547 uses a multi-wire feedthrough having a ceramic core with a plurality of grooves along its outside length with the wires inserted in the grooves. The wires, either tungsten or molybdenum, are twisted together at least at one end of the feedthrough. The twisted wire may be used as the electrode inside the lamp or a separate electrode tip may be attached to the twisted wire bundle.